The impact of Mycobacterium tuberculosis (MTB) infection has risen alarmingly world wide and in the US. At increased risk are patients with immunodeficiencies, including those with HIV, diabetes, cancer transplant recipients, and the elderly. Presently, the diagnosis of MTB infection relies on skin tests using tuberculin antigen (TST), which routinely fails in immunodeficient individuals, the population at primary risk. Moreover, the TST does not distinguish between immunity induced by vaccination with BCG or infection with MTB. Because immune defense against MTB primarily relies on T cell-mediated immunity, unlike most other infections, direct measurements of specific antibodies have proven to be of limited value in diagnosing MTB. Here we propose to develop a simple diagnostic kit that detects MTB-induced, MTB-specific memory T cells directly ex vivo, in venous blood. A highly sensitive, new generation cytokine ELISPOT assay will be adopted for this purpose. In aim 1, we will test whether a 240 kD antigen of MTB that is not expressed in BCG can distinguish between infected and vaccinated individuals. In aims 2 and 3, we will test whether the initial studies in Phase I suggest that direct testing of T cell memory can overcome the limitations of skin testing (false negative results with immunodeficient individuals/false positive with vaccinated persons). In Phase II we would like to develop a diagnostic kit that might complement or replace present TST. With millions of TST done annually in the US alone, such a kit should find a considerable market. PROPOSED COMMERCIAL APPLICATION: Diagnostic kit for M. tuberculosis.